2 cents here.. I currently have a lvl 55 shadow priest.  I found that leveling to around 50 was fast and painless...easy grinding easy leveling.  At 50 I found that i was still wearing armor from the mid to late 30's.  Specced disc/holy (disc to get imp spirit) and ran lvl 50 instances for 2 levels to get all blue gear.  Got a whole new set of gear, and respecced shadow...my grinding and leveling pace almost doubled with the new gear.  The other thing is that i got a taste of holy and liked it...if you haven't gone holy/disc, it is worth the time and effort. As I say this I also want to emphasize--I HAD NO PROBLEM HEALING ANY INSTANCE AS SHADOW--but the difference in healing between holy/disc and shadow is VERY noticeable.  I think if you try a holy/disc build at a relatively high level, you'll find out if you like it or not within a few hours.